The use of powdered metals, and particularly iron and its alloys, is known for forming magnets, such as soft magnetic AC cores for transformers, inductors, motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression, injection molding, or isostatic pressing techniques, can be used to form intricate molded part configurations, such as magnetic cores, without the requirement for additional machining and/or piercing operations. As a result, the formed part is often substantially ready for use within its working environment as formed by the molding process.
Molded soft magnetic cores for AC applications generally should have low magnetic core losses. To provide low core losses, the individual metal particles within the magnetic core must be electrically insulated from each other. Numerous types of insulating materials, which also act as the binder required for molding, have been suggested by the prior art, including inorganic materials such as iron phosphate and alkali metal silicate, as well as an extensive variety of organic polymeric materials. It is also known to coat a powdered metal with an inorganic undercoating and then provide an organic topcoat. In addition to providing adequate insulation and adhesion between the metal particles upon molding, the coating material should also have the ability to provide sufficient lubrication during the molding operation so as to enhance the flowability and compressibility of the particles, therefore enabling the particles to attain maximum density and strength.
A shortcoming of the prior art arises in that a magnetic core's maximum operating temperature will often be determined by the heat resistant properties of the insulating material used to adhere the metal particles together. It is essential that the integrity of the insulating material be maintained so as to insulate the individual metal particles and thereby provide low core losses for AC applications. If the magnetic core is exposed to a temperature which exceeds the degradation temperature of the coating material, the ability of the coating material to encapsulate and adhere the particles will likely be degraded, which could ultimately destroy the magnetic core. Even where physical destruction of the magnetic core does not occur, the magnetic field characteristics of the magnetic core will likely be severely impaired because of the degradation of the insulating capability of the coating material due to the elevated temperatures.
As disclosed in copending U.S. patent application Ser. No. 07/976,859, filed Nov. 16, 1992, which is assigned to the assignee of the present invention, polybenzimidazole (PBI), aromatic polyamides such as polyphthalamide (PPA), and certain polyimides have been found to perform well as the coating material for powdered iron and/or powdered iron alloys. Each of these preferred thermoplastic polymers have operating temperatures, as defined by their heat deflection temperatures, which permit their use in high temperature applications of greater than about 270.degree. C. As a result, these preferred polymers perform well, particularly with respect to their ability to withstand relatively high operating temperatures such that the mechanical properties and desired magnetic characteristics of the molded magnetic core do not deteriorate at high temperatures.
Polybenzimidazole, an aromatic polyamide such as polyphthalamide, and the preferred polyimides also have the ability to adhere well to the underlying iron particle, bind the iron particles together, and resist thermal and chemical attack, while also serving as a lubricant during the compression molding process so as to promote high density and strength of the magnetic core. The ability of an encapsulating material to serve as a lubricant during the molding process is also important in that unsuitably low densities correspond to a lower magnetic permeability of the magnetic core.
However, a shortcoming associated with compression molded magnetic cores is that work hardening of the metal particles occurs during the compression molding process, inducing stresses within the magnetic cores that result in reduced magnetic permeability, increased coercivity and possibly higher core losses. As an example, the magnetic permeability of magnetic cores formed by conventional compression molding techniques typically does not exceed about 125 Gauss/Oersteds (G/Oe) at about 50 oersteds field intensity and about 100 to about 400 Hz. As a result, magnetic cores which are compression molded from encapsulated iron particles generally do not exhibit sufficiently high magnetic permeability to be useful in AC applications such as generators, stator cores, transformers and the like, which require magnetic permeability in excess of about 175 G/Oe as measured at about 50 oersteds field intensity and about 100 to about 400 Hz. Moreover, work hardening of the metal also increases the coercivity of soft magnetic cores. Coercivity is that property of the metal that is measured by the maximum value of coercive force (H.sub.c) which is the magnetizing force required to bring the induced magnetism to zero in a magnetic material which is in a symmetrically cyclically magnetized condition. High coercivity is undesirable in soft magnets because energy is wasted bringing the induced magnetism back to zero. Moreover, in low frequency (i.e., less than about 200 Hz) applications most of the core losses are attributable to hystersis losses which are controlled by the metal component of the core and can only be reduced by attention to the metal component. Stress relieving the metal component is one way to reduce its coercivity.
To relieve the undesirable stresses induced into the metal particles during compression molding, it would be necessary to anneal a magnetic core at a temperature of at least about 450.degree. C., and then cool the magnetic core without quenching. However, polymer coatings generally cannot withstand such temperatures, and tend to degrade and pyrolyze, causing a significant loss of strength and magnetic properties in the magnetic core.
Thus, it would be desirable to provide a method for enhancing the magnetic permeability of compression molded magnetic cores which are formed from encapsulated powdered metals, wherein the coating material has the ability to withstand processing temperatures which are sufficient to anneal the magnetic core so as to relieve the stresses induced by the compression molding process. Furthermore, it would be desirable that such a method not cause a corresponding loss in the mechanical properties and magnetic characteristics of the molded magnetic core as a result of the degradation and/or pyrolyzation of the coating material during annealing. In addition, such a coating material should be soluble in a suitable solvent, and capable of improving lubrication during the molding process and providing adhesion between the metal particles after molding, so as to attain maximum density and strength of the as-molded article.